The United States and the European Union urged the Armenian authorities on Tuesday to investigate major irregularities reported during the weekend referendum, saying that is essential for the legitimacy of President Serzh Sarkisian’s constitutional reform.

The U.S. Embassy in Yerevan was particularly concerned about the conduct of the referendum, describing allegations of vote rigging as “credible.”

“The credible allegations of electoral irregularities reported by both non-partisan observers as well as Armenian political parties are of concern, however, and need to be fully investigated to ensure that the Armenian people can see the outcome of the referendum as credible and legitimate,” the embassy stressed in a statement.

“We urge the Electoral Commission and the Armenian government to carry out transparent investigations of all credible reports of irregularities,” it said.

“We encourage the authorities to fully investigate fraud allegations, for the referendum process to be credible,” Ambassador Piotr Switalski, the head of the EU Delegation to Armenia, said in written comments to RFE/RL’s Armenian service (Azatutyun.am).

Switalski said at the same time that the EU “took note” of the official referendum results indicating a “Yes” vote for Sarkisian’s controversial constitutional changes. “Is now important to implement the new constitution, in particular the human rights chapter, the new checks and balances and minority rights for the opposition, so that it does not just remain on paper,” he added.

Both Switalski and the U.S. Embassy said the authorities should enact a new Electoral Code meeting democratic standards in advance of Armenia’s next parliamentary elections due in May 2017. The authorities must also ensure that the elections are “viewed by the Armenian people as credible, legitimate, and a true reflection of their will,” stressed the embassy statement.

The 2017 elections will determine who will govern Armenia after President Serzh Sarkisian completes his second and final term in 2018. The constitutional amendments envisage the country’s transformation into a parliamentary republic with a largely ceremonial president.

The U.S. and the EU have not formulated a position on those changes throughout the constitutional reform process which Sarkisian launched over two years ago. Nor did the EU send a monitoring mission to Sunday’s referendum marred by fraud allegations. Richard Mills, the U.S. ambassador to Armenia, monitored voting in some polling stations in Yerevan.

On Monday, the EU began official negotiations with Armenia on a new agreement to deepen their political and economic relations. Foreign Minister Edward Nalbandian cited the official referendum results when he spoke at the start of the talks in Brussels. Nalbandian described Sarkisian’s amendments as “another important step” towards democratic change in Armenia.

Armenian opposition groups insist, however, that the main purpose of the reform is to enable Sarkisian to extend his rule beyond 2018.